1. Field of the Invention
The present invention relates to low-temperature service nickel steel plates, each with excellent weld toughness, and particularly to such steel plates for use in fabricating the bases and rooftops of a liquefied natural gas (LNG) container tanks, and, as well as those steel plates for use in building inboard tanks of liquefied natural gas container ships. Because these applications involve exposure to extra low temperatures below -160.degree. C., weld toughness is of vital significance.
2. Description of the Related Art
To date, 9% Ni steel has been a typical category for the production of LNG tank fabricating steel plates. Particularly, of such steel plates, a thin type has heretofore usually been selected, reflecting the fact that both the base and rooftop do not undergo heavy stress. In such applications, where the steel plates are exposed to extra low temperatures below -160.degree. C., high toughness especially at welds is required.
Generally, the respective regions which are thermally affected within each weld of a steel plate are classified into the following:
(a) A coarse grain region to undergo intensive heating at a temperature of approximately over 1200.degree. C.
(b) A fine grain region to go through with heating over a range of approximately 1200.degree. to 900.degree. C.
(c) A dual phase region to incur heating over a range of approximately 900.degree. to 700.degree. C.
(d) A tempered region to suffer heating over a temperature range of approximately 700.degree. to 450.degree. C.
Of these regions which are thermally affected in the course of welding, the coarse grain region which experiences intensive heating, has so far been given various measures to maintain its toughness.
Of late, however, it was found that the dual phase region within a weld joint between two thin steel plates incurred some loss of toughness (dual phase embrittlement) induced by the formation of island martensite, thereby causing a problem. It was noted that the embrittlement due to dual phase was conspicuous with high Ni-content steel which is of more than 7.5 wt % Ni (hereinafter, the Ni-content is presented in percentage). The reason behind this was subsequently clarified by the inventor through a study which showed that the island martensite would readily be generated at a dual phase region post-heating cooling stage due to a difference in carbon grain dispersion between austenite and ferrite in the dual phase region.
In this regard, it is known that tempering is useful in dissolving the island martensite. With this in view, it is understood that where two thick steel plates are welded together in a large number of passes, the island martensite once generated is partly dissolved in a subsequent welding heat cycle (equivalent to tempering), with the apparent loss of dual phase regional toughness failing to take place. Regrettably, however, it was disclosed by the inventor, et al, that for a weld joint between each two thin steel plates each measuring a thickness of not more than 10 mm, the number of passes required is not more than three. The island martensite is left totally undissolved after a subsequent welding heat cycle, whereby the dual phase region undergoes an outstanding toughness loss, with an increase in brittleness.
Accordingly, the inventor, et al, proposed in Japanese Unexamined Patent Publication No. 63-290246 as a measure to overcome this problem, that Ti would be added as an essential element with Mo selectively added as an optional element, upon optimally decreasing the contents respectively of Si and Mn. Further studies thereafter undertaken by the inventor brought forth the finding that the method referred to in this proposal was useful to improve toughness of a heating-inflicted thermally affected dual phase region, but not useful with a heating-inflicted thermally influenced fine grain region, where the fine grain region would show a brittle fracture surface in the Charpy impact test carried out at -196.degree. C., whereby it turned out that the toughness of said fine grain region rather reduced.
In addition to the above-described method, there are also available other techniques which are disclosed respectively in Japanese Unexamined Patent Publication No. 63-128118, Japanese Patent Publication No. 56-10966, and Japanese Unexamined Patent Publication No. 56-156716. These techniques were devised solely to improve low-temperature toughness of a base metal, and via a study, the inventor, et al, ascertained that they were not very useful for the prevention of a dual phase regional toughness loss and avoidance of embrittlement.